Disposable paper towels, napkins, and facial tissue are absorbent structures that need to remain strong when wet. For example, paper towels need to retain their strength when absorbing liquid spills, cleaning windows and mirrors, scrubbing countertops and floors, scrubbing and drying dishes, washing/cleaning bathroom sinks and toilets, and even drying/cleaning hands and faces. A disposable towel that can perform these demanding tasks, while also being soft, has a competitive advantage as the towel could be multi-purpose and be used as a napkin and facial tissue. The same can be said about a napkin or facial tissue, which could become a multi-purpose product if the right combination of quality attributes can be obtained of which wet strength, absorbency, and softness are key attributes.
The industrial methods or technologies used to produce these absorbent structures are numerous. The technologies that use water to form the cellulosic (or other natural or synthetic fiber type) webs that comprise the towel or wipe are called Water-Laid Technologies. These include Through Air Drying (TAD), Uncreped Through Air Drying (UCTAD), Conventional Wet Crepe (CWC), Conventional Dry Crepe (CDC), ATMOS, NTT, QRT and ETAD. Technologies that use air to form the webs that comprise the towel or wipe are called Air-Laid Technologies. To enhance the strength and absorbency of these towels and wipes, more than one layer of web (or ply) can be laminated together using strictly a mechanical process or preferably a mechanical process that utilizes an adhesive.
Absorbent structures can be produced using both Water or Air-Laid technologies. The Water-Laid technologies of Conventional Dry and Wet Crepe are the predominant method to make these structures. These methods comprise forming a nascent web in a forming structure, transferring the web to a dewatering felt where it is pressed to remove moisture, and adhering the web to a Yankee Dryer. The web is then dried and creped from the Yankee Dryer and reeled. When creped at a solids content of less than 90%, the process is referred to as Conventional Wet Crepe. When creped at a solids content of greater than 90%, the process is referred to as Conventional Dry Crepe. These processes can be further understood by reviewing Yankee Dryer and Drying, A TAPPI PRESS Anthology, pg 215-219 which is herein incorporated by reference. These methods are well understood and easy to operate at high speeds and production rates. Energy consumption per ton is low since nearly half of the water removed from the web is through drainage and mechanical pressing. Unfortunately, the sheet pressing also compacts the web which lowers web thickness and resulting absorbency.
Through Air Drying (TAD) and Uncreped Through Air Drying (UCTAD) processes are Wet-Laid technologies that avoid compaction of the web during drying and thereby produce absorbent structures of superior thickness and absorbency when compared to structures of similar basis weight and material inputs that are produced using the CWP or CDC process. Patents which describe creped through air dried products include U.S. Pat. Nos. 3,994,771, 4,102,737, 4,191,609, 4,529,480, 467,859, and 5,510,002, while U.S. Pat. No. 5,607,551 describes an uncreped through air dried product.
The remaining Wet-Laid processes termed ATMOS, ETAD, NTT, STT and QRT can also be utilized to produce absorbent structures. Each process/method utilizes some pressing to dewater the web, or a portion of the web, resulting in absorbent structures with absorbent capacities that correlate to the amount of pressing utilized when all other variables are the same. The ATMOS process and products are documented in U.S. Pat. Nos. 7,744,726, 6,821,391, 7,387,706, 7,351,307, 7,951,269, 8,118,979, 8,440,055, 7,951,269 or U.S. Pat. Nos. 8,118,979, 8,440,055, 8,196,314, 8,402,673, 8,435,384, 8,544,184, 8,382,956, 8,580,083, 7,476,293, 7,510,631, 7,686,923, 7,931,781, 8,075,739, 8,092,652, 7,905,989, 7,582,187, 7,691,230. The ETAD process and products are disclosed in U.S. Pat. Nos. 7,339,378, 7,442,278, and 7,494,563. The NTT process and products are disclosed in international patent application WO 2009/061079 A1 and U.S. Patent Application Publication Nos. US 2011/0180223 A1 and US 2010/0065234 A1. The QRT process is disclosed in U.S. Patent Application Publication No. 2008/0156450 A1 and U.S. Pat. No. 7,811,418. The STT process is disclosed in U.S. Pat. No. 7,887,673.
To impart wet strength to the absorbent structure in the wet laid process, typically a cationic strength component is added to the furnish during stock preparation. The cationic strength component can include any polyethyleneimine, polyethylenimine, polyaminoamide-epihalohydrin (preferably epichlorohydrin), polyamine-epichlorohydrin, polyamide, or polyvinyl amide wet strength resin. Useful cationic thermosetting polyaminoamide-epihalohydrin and polyamine-epichlorohydrin resins are disclosed in U.S. Pat. Nos. 5,239,047, 2,926,154, 3,049,469, 3,058,873, 3,066,066, 3,125,552, 3,186,900, 3,197,427, 3,224,986, 3,224,990, 3,227,615, 3,240,664, 3,813,362, 3,778,339, 3,733,290, 3,227,671, 3,239,491, 3,240,761, 3,248,280, 3,250,664, 3,311,594, 3,329,657, 3,332,834, 3,332,901, 3,352,833, 3,248,280, 3,442,754, 3,459,697, 3,483,077, 3,609,126, 4,714,736, 3,058,873, 2,926,154, 3,877,510, 4,515,657, 4,537,657, 4,501,862, 4,147,586, 4,129,528 and 3,855,158.
Absorbent structures are also made using the Air-Laid process. This process spreads the cellulosic, or other natural or synthetic fibers, in an air stream that is directed onto a moving belt. These fibers collect together to form a web that can be thermally bonded or spray bonded with resin and cured. Compared to Wet-Laid, the web is thicker, softer, more absorbent and also stronger. It is known for having a textile-like surface and drape. Spun-Laid is a variation of the Air-Laid process, which produces the web in one continuous process where plastic fibers (polyester or polypropylene) are spun (melted, extruded, and blown) and then directly spread into a web in one continuous process. This technique has gained popularity as it can generate faster belt speeds and reduce costs.
To further enhance the strength of the absorbent structure, more than one layer of web (or ply) can be laminated together using strictly a mechanical process or preferably a mechanical process that utilizes an adhesive. It is generally understood that a multi-ply structure can have an absorbent capacity greater than the sum of the absorbent capacities of the individual single plies. It is thought this difference is due to the inter-ply storage space created by the addition of an extra ply. When producing multi-ply absorbent structures, it is critical that the plies are bonded together in a manner that will hold up when subjected to the forces encountered when the structure is used by the consumer. Scrubbing tasks such as cleaning countertops, dishes, and windows all impart forces upon the structure which can cause the structure to rupture and tear. When the bonding between plies fails, the plies move against each other imparting frictional forces at the ply interface. This frictional force at the ply interface can induce failure (rupture or tearing) of the structure thus reducing the overall effectiveness of the product to perform scrubbing and cleaning tasks.
There are many methods used to join or laminate multiple plies of an absorbent structure to produce a multiply absorbent structure. One method commonly used is embossing. Embossing is typically performed by one of three processes: tip to tip, nested, and/or rubber to steel embossing. Tip to tip embossing comprises axially parallel jumbo rolls of the absorbent structure juxtaposed to form a nip between the crests of the embossing tips of the opposing emboss rolls. The nip in nested embossing has the embossing tips on one emboss roll meshed between the embossing tips of the other. Rubber to steel embossing comprises a steel roll with embossing tips opposed to a roll having an elastomeric roll cover wherein the two rolls are axially parallel and juxtaposed to form a nip where the embossing tips of the emboss roll mesh with the elastomeric roll cover of the opposing roll.
For example, during the tip to tip embossing process of a two ply absorbent structure web, each web is fed through separate nips formed between separate embossing rolls and pressure rolls with the embossing tips on the embossing rolls producing compressed regions in each web. The two webs are then fed through a common nip formed between the embossing rolls where the embossing tips on the two rolls bring the webs together in a face to face contacting relationship.
By comparison, nested embossing works by having the crests of the embossing tips on one embossing roll intermesh with the embossing tips on the opposing embossing roll with the nip formed between the two rolls. As the web is passed between the two embossing rolls, a pattern is produced on the surface of the web by the interconnectivity of the tips of one roll with the open spaces of the opposing roll.
Rubber to steel embossing works by having one hard embossing roll with embossing tips in a desired pattern and a back-side soft impression roll, often having an elastomeric roll cover aligned in an axially parallel configuration to form a nip between the rolls. As the web is passed through the nip between the rolls, the embossing tips impress the web against and into the rubber to deform the structure of the web.
It is possible to marry two or more webs of an absorbent structure (or different absorbent structures) together using an adhesive. In an exemplary nested embossing process an adhesive applicator roll may be aligned in an axially parallel arrangement with one of the two embossing rolls forming a nip therewith, such that the adhesive applicator roll is upstream of the nip formed between the two embossing rolls. The adhesive applicator roll transfers adhesive to the embossed webs on the embossing roll at the crests of the embossing knobs. The crests of the embossing knobs typically do not touch the perimeter of the opposing roll at the nip formed there between, necessitating the addition of a marrying roll to apply pressure for lamination. The marrying roll forms a nip with the same embossing roll forming the nip with the adhesive applicator roll, downstream of the nip formed between the two embossing rolls. An example of this lamination method is described in U.S. Pat. No. 5,858,554.
Other attempts to laminate absorbent structure webs include bonding the plies at junction lines wherein the lines include individual pressure spot bonds. The spot bonds are formed by the use of thermoplastic low viscosity liquid such as melted wax, paraffin, or hot melt adhesive, as described in U.S. Pat. No. 4,770,920. Another method laminates webs of absorbent structure by thermally bonding the webs together using polypropylene melt blown fibers, as described in U.S. Pat. No. 4,885,202. Other methods use metlblown adhesive applied to one face of an absorbent structure web in a spiral pattern, a stripe pattern, or random patterns before pressing the web against the face of a second absorbent structure, as described in U.S. Pat. Nos. 3,911,173, 4,098,632, 4,949,688, 4,891249, 4,996,091 and 5,143,776.